They extends the regular atom entries, by adding 2 nodes: verbs and object. The author information can also be extended.
With this, it is possible to represent almost any action done by a specific user: Julien commented on a blog post. Astro was tagged in a picture. John friended Andy.

Soon, we will start to extrapolate ActivityStreams data out of non-as data, and then, all the data that you’ll get from Superfeedr will be ActivityStream!